Patent Application
20070024298
The present invention relates generally to the testing of semiconductor chips, and specifically to the design of probe contactors for such testing prior to packaging.
Typically, semiconductor chips are tested to verify that they function appropriately and reliably. This is often done when the semiconductor devices are still in wafer form, that is, before they are diced from the wafer and packaged. This allows the simultaneous testing of many chips at a single time, creating considerable advantages in cost and process time compared to testing individual chips once they are packaged. If chips are found to be defective, then when the chips are diced from the wafer, the defective ones can be discarded and only the reliable chips are packaged. It is an axiom then that the larger a wafer that may be reliably tested at a time, the more savings can be incurred in cost and process time.
Generally, when performing wafer testing, a chuck carrying a wafer is raised to a probe card to which thousands of probes are electrically coupled. To test larger wafers, small, high performance probes are needed. The probes must be able to break through the oxide and debris layers on the surface of the contact pads of the chips on the wafer in order to make a reliable electrical contact to each pad. Additionally, the probes must be able to compensate for the fact that the contact pads may be of different heights (i.e., not all the contact pads on a wafer may reside in the same plane). Furthermore, the chuck and the probe card mechanical mount may not be precisely parallel and flat, introducing further height variations which the probes must accommodate.
Conventionally, cantilever wire probes have been used to test wafers in this regard. However, cantilever wire probes are too long and difficult to accurately assemble to allow reliable simultaneous contact to all of the chips on a conventional wafer. Additionally, cantilever wire probes have high self and mutual inductance problems which do not make them good candidates for testing of high-speed devices. These problems are exaggerated when they are used to test larger wafers. Cantilever (or bending) probes can also be fabricated at a small physical scale by various mircofabrication techniques known in the art. These cantilever springs lack the mechanical energy density (for controlled scrubbing of the oxide layer) and spatial efficiency to be ideally effective for reliable testing of large wafers.
A number of attempts have been made to overcome the deficiencies of cantilever probes, all with varied levels of success. For instance, U.S. Pat. No. 5,926,951, assigned to Form Factor, Inc., describes methods of producing spring probes by coating a ductile metal with a spring metal (as seen in
U.S. Pat. No. 6,426,638, assigned to Decision Track, describes a torsion spring design, see
Improvements in the design of probe contactors have come with advances in photolithography and associated micromachining techniques. U.S. Pat. No. 5,190,637 to Wisconsin Alumni Research Foundation describes the basis of multi-layer build up fabrication through lithographic electro-forming techniques of three-dimensional metal structures including springs and spring contactors. The present applicants have created a micro-formed torsion bar probe contactor which overcomes many of the deficiencies of the prior art and is a subject of the instant application.
Another aspect of the present application is the formation of the tip at the end of the probe. Older pin based contactors, such as cantilever needle probes or vertically buckling beam probes, are typically built from wire with a sharpened or shaped tip. This type of geometry provides for adequate electrical contact only if substantial contact force is applied. High contact force is deleterious to the semiconductor devices under test which often include active devices under the I/O pads. Furthermore, pin based contactors cannot be built at the fine pitches and high pin counts required for modern large wafer test. For these and other reasons, microfabricated probe contactors are an attractive alternative to pin based probe cards.
Microfabricated probe contact tips for use on contactor probes have been proposed in a variety of configurations and are plentiful in the art. In most of these configurations, provision is made for the creation of a tip with a well defined and controlled surface shape, size, material, and texture. Each of these elements is important for achieving the required consistent electrical contact to common IC pad metals such as Al, AlSiCu, Cu, Cu alloys, Au, or solder. Each of these parameters has a bearing on the contact performance but control over the geometry is among the most significant and is a function of the fabrication technology employed.
Another factor that is often overlooked is the optical characteristic of the tip and adjacent structures. Typically, probe cards are used in conjunction with wafer probers equipped with machine vision systems for automatic identification of probe tip locations and alignment of those to the I/O pads on the wafer, such as that described by U.S. Pat. No. 5,321,352, assigned to Tokyo Electron Labs. Basically, a machine vision system includes a camera that is positionable and looks at the tips of the probe needles. The camera has some magnification appropriate for viewing the geometry of the tip. It also includes a light source such as an LED ring light or a coaxial light. The image from the camera is processed by computer so as to determine the location of the tip relative to the camera's image area. This location information is used by the prober's computer control algorithm to position the DUT bond pads accurately under the probe tips. Thus the probe tip must be designed with the vision systems requirements in mind. In particular, vision systems require a good optical contrast between the tip and adjacent structures, particularly in the case of microfabricated contactors with small physical dimensions between adjacent surfaces. Typical microfabricated spring contactors have smooth planar surfaces in close proximity to the contact tip surface, creating difficulty with regard to the vision recognition systems due to reflections from surfaces other than the tip, as seen in
Various attempts to overcome this problem have been suggested, but each have had their own problems. For instance, U.S. Pat. No. 6,255,126, assigned to Form Factor, Inc. and shown in
Another way of solving machine vision problems is to create the tip significantly tall (approximately 50 um tall). In this embodiment, the next underlying planar surface (the post) would be far enough away from the focal plane of the vision microscope so that the post surface would be out of focus and only the tip would be in focus. However, this is not a practical solution for tips that are produced by lithographic imaging and electroforming. Such processes have practical limits in aspect ratio (height to width ratio). Furthermore, even if the aspect ratios of a taller tip were practical (typical tips are about the same height as or slightly higher than their smallest dimension which is on the order of 5 um to 20 um), a taller tip would be prone to breakage from the lateral scrubbing forces present in use.
Another proposed alternative is to remove part of the post structure, creating a sloped surface around the tip, see
Thus a new design is needed for creating a tip and post structure that will resolve the issues of vision errors when a tip is lithographically formed on a probe structure.
Improvements upon the lithography techniques described in U.S. Pat. No. 5,190,637 are the subject of U.S. patent application Ser. Nos. 11/019,912 and 11/102,982, both commonly owned by the present applicant and hereby also incorporated by reference. Those two applications describe the use of general photolithographic pattern-plating techniques combined with the use of islands of sacrificial metals to further create microstructures such as probe contactors. Using the above techniques, the present applicants have created a micro-formed torsion bar probe contactor which overcomes many of the deficiencies of the prior art and that is a subject of the instant application.
The present invention is directed to a probe incorporating a torsion bar as a spring element and a tip and post structure that resolves current problems with automatic vision mechanisms. The torsion bar probe is formed on a substrate which will ultimately hold hundreds or thousands of probe elements. The probe is connected to the substrate by a foot. Attached to one end of the foot is a trace that electrically connects the foot to a via in the substrate, and at the other end of the foot, a torsion bar is attached. At the other end of the torsion bar, a spacer is attached, the spacer being taller than the torsion bar. Atop the spacer, an arm is attached. The arm is more rigid than the torsion bar, meaning that it does not significantly bend to store energy during use. Atop the arm, opposite the spacer, a post (or posts) are attached, and atop the post(s) is a tip, the structure of which will be described further below. A stop is built atop the substrate at a place near and below where the spacer and torsion bar are joined. There is a space or a gap between the torsion bar and the stop when the probe is in a non-actuated state (i.e., not pressed against a contact pad of a semiconductor device).
In operation, the tip is contacted by an I/O contact pad on a wafer and forced down (in the spatial orientation of the majority of the drawings) towards the substrate. As the tip is pushed down, the arm, which is designed to be mostly rigid, tilts causing the torsion bar to twist. The torsion bar is firmly affixed to the substrate at the foot end, and is supported both vertically and laterally, but free to rotate, at the stop end. Rotation at the stop end involves a slight motion of the torsion bar (through the gap distance of a few microns) until contact is made to the stop, after which point the torsion bar pivots against the stop. The overall geometry of the probe (including spacer height, arm length, post height, etc.) dictates the motion in space of the tip as it travels downward. The motion is largely in the form of an arc, providing a forward component (in a direction roughly orthogonal to the axis of the torsion bar) as the tip moves downward. The forward motion of the tip provides the “scrub” which is necessary in practice to achieve a good reliable and repeatable contact resistance to the I/O pad.
Utilizing the manufacturing processes described in U.S. patent application Ser. Nos. 11/019,912 and 11/102,982, the present invention includes several novel features not present in the prior art. One of the novel features of the present invention is that the arm portion is in a different planar layer than the torsion bar and may be separated from the torsion bar by a spacer. The addition of these two added layers provides for greater design flexibility towards controlling the path of motion of the tip when it is actuated by a largely vertical force when testing a device. The availability of additional layers of the probe in this respect is made possible by the new manufacturing processes described in the above patent applications. In fact, as described, the torsion probe has at least eight planar layers utilized in the construction of the torsion contactor spring and these layers afford design flexibility to optimize the operational characteristics of the contactor while accommodating the process limitations imposed by commercially viable photolithographic micro-electro-forming techniques. The torsion probe may have more or fewer layers than eight without departing from the spirit of this invention.
The arm of the probe is also made more rigid than the torsion bar so that it does not act as a spring (as in a cantilever beam spring). If the bar was not rigid, its deformation would increase the scrub length beyond that which may be desired. The arm provides a lever which, in consort with the stop, transforms the largely linear arc of the tip to a nearly pure-torsion rotation of the torsion bar. In another embodiment, the arm is composed of two subarms, one extending from the top of the end of the other. This approach allows a greater clearance between the closest part of the spring structure and the wafer under test. Additional clearance helps to avoid damage to the wafer from foreign particles that may become caught between the probe structures and the wafer in the contacted state.
Another novel aspect of the present invention is that a stop is attached to the substrate and incorporates a lateral support to laterally support the torsion bar when it engages the stop. The basic function of the stop is to act as a fulcrum or pivot for the torsion bar. The lateral support provides an increase in lateral stability and more control over the tip's scrub pattern.
Another novel aspect of the present invention is the design of the tip and post structure. To ensure that the machine vision systems can accurately differentiate the tip from the post when both have planar surfaces, the top portion of the post, to which the tip is coupled, is treated in such a way that it diffuses or absorbs incident light. This can be accomplished several ways, such as through rough plating (such as with high current plating with or without the addition of grain refiners) or metallographic decorative etching.
A further refinement is to provide a rough plated skirt that covers not only the top surface of the support post, but also wraps around the base of the tip. This construction not only creates a high contrast between the tip surface and the post but also provides for mechanical support of the tip in the form of a thickening or gusset around its base. The gusset further protects the tip from mechanical failure at its base caused by lateral forces during use, which is particularly useful if the tip has a high aspect ratio (height to width or diameter). This skirt may also be plated in a pattern that slightly overhangs the post structure so that slight misalignments (e.g., due to lithography errors) do not result in the exposure of the smooth reflective surface in the shape of a crescent or edge at one side of the perimeter of the post.
At the distal end (again, the side that is furthest from the via 900) the torsion bar 630 is coupled to a spacer element 640. The spacer element raises the arm element 650 off from the plane of the torsion bar 630. The spacer element 640 provides design flexibility towards controlling the path of the motion of the tip 680. It also provides the clearance required between the arm 650 and the substrate 600 to accommodate a full range of compliance. On top of the spacer 640, is the arm element 650. The arm element 650 is plated generally non-axial to the torsion bar 630. In one embodiment, the arm 650 is at an angle of about 20 to about 160 degrees relative to the torsion bar 630, and preferably the angle is about 120 degrees. The arm 650 is designed to be rigid so that it does not act as a spring. If the arm 650 were especially flexible, its deformation would contribute to increasing the scrub beyond the desired limit. In this regard, the arm 650 may be made of a higher modulus metal (for example, W) than the torsion bar 630 (formed from, for example, NiMn), or it may be made shorter, thicker (increasing the height) or wider (or any combination) than the torsion bar 630 as in the preferred embodiment. In this regard, the length, thickness, and width can be expressed in the three axes of three-dimensional Cartesian coordinates, conventionally denoted the x, y, and z axis. The x-axis represents the length, the y-axis represents the width, and the z-axis represents the thickness.
The fact that the arm 650 is on a different plane than the torsion bar 630 is a novel feature of the present invention. This feature is made possible by the use of the photolithography process described in U.S. application Ser. Nos. 11/019,912 and 11/102,982, which are incorporated herein by reference.
Atop the distal end of the arm 650, a first post 660 is plated. The first post element 660 also provides design flexibility in controlling the path of the motion of the tip 680 during actuation. The first post 660 may have a tip 680 plated on top of it, or there may be a second (or more) post element(s) 670, optionally having a smaller surface area than the first post element 660, plated between the first post 660 and the tip 680. The post element (either a first post 660 or combined with the secondary post element 670) extends the tip 680 vertically away from the arm 650 allowing the full target deflection of the tip. The secondary post 670 may be added to the first post 660 to allow proper geometries for tip scrub while maintaining manufacturability. The first post 660 may be plated large enough to allow lithography and plating with a roughly (or slightly larger) 1:1 aspect ratio (width to height). The secondary post 670 is ideally smaller in order to more adequately accommodate a proper scrub. A smaller secondary post 670 also accommodates lithographic alignment errors between the two post layers.
The tip 680 does not need to be concentric with whichever post (660 or 670) it is plated on. It may be advantageous to plate the tip 680 offset from the center of the post (660 or 670) upon which it is plated in order to eliminate any interference the post (660 or 670) may have with the device under test due to the deflection angle. The tip 680 may be circular, rectangular, blade-shaped, oval, teardrop shaped, or any other shape that can be formed lithographically.
Underneath the arm 630 is a stop element 690. The stop element 690 is plated on the substrate 600 and there is a gap 910 (see
One could increase the height of the posts 660, 670 in order to create greater clearance between the wafer surface and the probe structure, but this is undesirable because it increases scrub length and adds process complexity and cost. While the embodiment shown in
In an embodiment of the present invention shown in
In the embodiment of the Figure of 6A, there may be a distance of 100 um from the top plane of the arm 650 and the top plane of the tip 680. There may also be a distance of 110 um between the top plane of the substrate 600 and the bottom plane of the arm 650. The total distance from the top plane of the tip 680 and the top plane of the substrate 600 may be 270 um.
In the embodiment of
While the forgoing dimensions give approximate dimensions for exemplary embodiments of the present invention, the actual dimensions may be varied by as much as ten times without significantly altering the design principles utilized. While Ni is utilized to make a majority of the probe element in the above examples, many other metals and metal alloys such as NiMn, Tungsten Alloys, and Cobalt alloys may also be used. In general, it is desirable to use metals that can be electroformed and that provide good mechanical strength, toughness and thermal stability.
The substrate 600 may be any type of substrate, including semiconductor materials such as silicon, germanium and gallium arsenide, ceramics such as alumina, aluminum nitride, glass bonded ceramics, low temperature cofired ceramics (LTCC) and high temperature cofired ceramics (HTCC), dielectric coated metals or glasses. The substrate 100 is preferably a Low Temperature Co-fired Ceramic (LTCC) substrate with built in vias 900 such that electricity may be conducted from one face 600a of the substrate 600 to the other face 600b of the substrate 600 by way of the vias 900. In an embodiment of the present invention, the vias 900 are made from gold, but any other conductor such as copper, tungsten or platinum may be used. The ceramic may also contain electrical redistribution conductors, making it an electrical wiring board or “space transformer” as is commonly known in the art.
Another novel feature of the present invention is the design of the tip and post structures on the probe. The quality and reliability of the electrical contact to a semiconductor I/O pad is a function of the tip material, tip size, tip geometry, scrub motion, and contact force. Each of these parameters has a bearing on the contact performance, but tip geometry is among the most significant and is a function of the fabrication technology employed. Older contactors, such as cantilever needle probes or vertical buckling beam probes were typically built from wire with a sharpened or shaped tip. However, this type of tip geometry is difficult to control at the micron scale and requires high contact force which is deleterious to the semiconductor devices under test. Furthermore, pin based contactors cannot be built at the fine pitches and high pin counts required for modern wafer test. For these and other reasons, microfabricated probe contactors are an attractive alternative. New microfabricated spring contactors often have smooth planar surfaces in close proximity to the contact surface which creates difficulty for automatic vision systems to easily identify the tip due to reflections from surfaces other than the tip. The new post and tip designs of the present invention overcome this common problem.
A further refinement to this idea is depicted in
While the description above refers to particular embodiments of the present invention, it should be readily apparent to people of ordinary skill in the art that a number of modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true spirit and scope of the invention. The presently disclosed embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description. All changes that come within the meaning of and range of equivalency of the claims are intended to be embraced therein.
